Optical communication systems typically handle optical pulses with bit rates in the range 2-10 Gbits/sec with future generations of systems being expected to a handle bit rates up to 40 Gbits/sec and beyond. At such higher bit rates, the onset of self phase modulation and cross phase modulation is known to occur at high power levels and it is important to be able to monitor transmissions to detect signal degradation associated with these effects. A problem associated with the tendency towards higher bit rates is that it becomes increasingly difficult to monitor signal degradation using methods which involve detection of the optical signal and subsequent electronic processing because of the inherent difficulties of performing such electronic processing at very high bit rates.
A telecommunications systems in which optical signals traverse an optical network will typically contain a combination of transmission waveguides, optical amplifiers, switches, cross-connects, filters and dispersion compensators. Monitoring of these elements of the system to detect failure or degradation would ideally require monitoring the optical signal at separate locations downstream of each of the elements such that deterioration or failure of any of the elements can be detected and localised to identify the specific elements responsible.
Presently, however, the provision of electronic detection and processing apparatus capable of eye measurement is prohibitively expensive for bit rates in excess of about 10 Gbits/sec and is therefore confined to wide band receivers which are typically downstream of a large number of such elements.
Examples of the use of performance monitors in such receivers to determine parameters related to the eye diagram are disclosed by Harman U.S. Pat. No. 4,097,697 and Tremblay et al U.S. Pat. No. 4,823,360. In each case, an optical signal is regenerated and signal quality monitored.
It is also known from H. Takara et al, "Eye--diagram measurement of 100 Gbits/sec optical signal using optical sampling": 22nd European Conference on Optical Communication--ECOC 1996 Oslo", to perform eye diagram measurement by optical sampling using an organic non-linear crystal. The ability to use optical sampling before conversion to electronic signal facilitates the use of less sophisticated electronic processing. Reliance upon the organic non-linear crystal however has inherent disadvantages such as a difficulty in integrating this optical component in a sensor system. A further major disadvantage of this method is that sampling pulses are required to be generated at high power, i.e. in excess of 200 watts, so that the method is useful only in the context of a laboratory oscilloscope.
It is known from Idler et al (IEEE Photonics Technology Letters, Vol. 8, No. 9, September 1996--"10 Gb/s Wavelength Conversion with Integrated Multiquantum-Well-Based 3-Port Mach-Zehnder Interferometer") to provide inversion of a single optical signal in addition to wavelength conversion by means of a Mach-Zehnder interferometer in which semiconductor optical amplifiers are utilised to set an interference condition between optical components of an input signal transmitted through first and second arms of the interferometer. A continuous wave optical signal propagated equally through the first and second arms is recombined to form an output signal which is modulated according to the interference condition and a pulsed optical signal is counterpropagated through only one of the arms so as to modulate the phase of one of the component signals by cross-phase modulation due to the non-linear characteristics of the semiconductor optical amplifier in that arm.
There remains a need for a more practical approach to eye measurement monitoring of optical signals in such systems.